linux-kernel - locking issue (hardirq+softirq+user)

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Date:	Wed, 20 Dec 2006 14:35:24 +0100
From:	Jiri Slaby <jirislaby@...il.com>
To:	Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: locking issue (hardirq+softirq+user)

Hi!

an user still gets NMI watchdog warning, that the machine deadlocked.

The code is something like this:

DEFINE_SPINLOCK(lock);

isr() /* i.e. hardirq context */
{
spin_lock(&lock);
...
spin_unlock(&lock);
}

timer() /* i.e. softirq context */
{
unsigned int f;
spin_lock_irqsave(&lock, f) /* stack shows, that it locks here */
...
spin_unlock_irqrestore(&lock, f)
...
mod_timer();
}

tty_open_or_whatever() /* i.e. user context */
{
unsigned int f;
spin_lock_irqsave(&lock, f)
...
spin_unlock_irqrestore(&lock, f)
}

init()
{
mod_timer();
request_irq();
register_that_open_with_something();
}

What's the correct locking approach in this situation? Is that correct (I tried
to go through Rusty Russel's guide to locking, but I didn't get it in this
case)? There were many spin_lock recursions in the driver
(drivers/char/isicom.c), which I removed, but it still deadlocks on SMP.

thanks,
-- 
http://www.fi.muni.cz/~xslaby/            Jiri Slaby
faculty of informatics, masaryk university, brno, cz
e-mail: jirislaby gmail com, gpg pubkey fingerprint:
B674 9967 0407 CE62 ACC8  22A0 32CC 55C3 39D4 7A7E
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